1. Field of the Invention
The present invention concerns a frequency converter for an asynchronous motor comprising a voltage converter equipped with switches controlled by a control circuit and powered by a DC bus on which there is a shunt measuring the DC current passing through the bus.
2. Discussion of the Background
In order to control a multiphase AC motor such as an asynchronous motor, a frequency converter is used which controls the motor at variable frequency and voltage from the AC network. This type of frequency converter comprises a rectifier stage which after filtering, supplies a DC power supply to a voltage converter. The converter comprises switches Sa, Sb, Sc which are controlled by a control circuit in order to output a series of fixed amplitude pulses (positive or negative) modulated in width, to the motor. This technique is called pulse width modulation (PWM).
In a balanced three-phase system, a variable may be represented by a vector in which the components on three axes at 120.degree. are values of this variable on each phase. The end of the output voltage vector Vs may be in one of eight positions; on the vertices of a hexagonal (vectors V1 to V6) and at the center of the hexagon (null vectors V0 and V7). There is a control combination of switches Sa, Sb, Sc for each position of the voltage vector.
The vectorial modulation technique consists of considering an arbitrary voltage vector Vs as being the combination of two adjacent vectors and the null vector within a time interval Tp.
Referring to FIG. 2, an output voltage vector Vs may be in any one of the six sectors formed in space by vectors V1 to V6. In the example shown in FIG. 2, Vs is located in sector 0 between adjacent vectors V1 and V2. Vector Vs may be controlled by its adjacent vectors, namely V1 and V2 in the case mentioned above. The application duration of each adjacent vector is determined by the projection of the vector Vs on the axis of this adjacent vector. These durations ti and tk are calculated as a function of the PWM period, the bus voltage and the angle .theta. made by Vs and the first adjacent vector.
Knowing the times ti and tk, the period Tp (the inverse of the switching frequency) is filled in symmetrically with the duration of null vectors V0 and V7. If a symmetric PWM is adopted, the switching diagram shown in FIG. 3 is obtained.
In an asynchronous motor, the statoral current generates the flux and the torque. The static current may be broken down into a magnetizing current Id which will generate the flux and an active current Iq which will generate the torque.
The phase currents Ia, Ib, and Ic can be restored by measuring the current Idc on the bus using a shunt and the statuses of switches Sa, Sb, and Sc shown in FIG. 1. The current Idc measured by the shunt is explained as a function of the phase currents Ia, Ib and Ic and as a function of the switching states of switches Sa, Sb, Sc (0 or 1) in the form Idc=Sa.Ia+Sb.Ib+Sc.Ic. Thus knowing the switching state of the bridge, it is theoretically possible to restore currents Ia, Ib, Ic in the phases starting from the current Idc in the bus. In order to do this, at least two states are necessary to identify two different currents per PWM period.
When the three reference voltages Va, Vb and Vc are close to zero, the duration of state such as V1 and V2 become small and it is impossible to correctly measure the bus current Idc. The duration of significant states must exceed a predetermined time TMIN that covers the inter-channel delay time, the current rise and setup time in the shunt, the current acquisition time and the processing time.
U.S. Pat. No. 4,520,298 concerns restoration of phase currents starting from the measured current in the DC bus, but does not consider the problem of restoring phase currents at low speed.
The paper "A Stator Flux-Oriented Voltage Source Variable Speed Drive Based On dc Link Measurement" published in IEEE in 1991, suggested canceling and accumulating states with durations that were too short until a minimum duration was obtained. At low speed this solution could cause large voltage oscillations.
The paper "Indirect Phase Current Detection for Field Oriented Control of a Permanent Magnet Synchronous Motor Drive" published by EPE in 1991, suggested replacing the duration of a state by a duration TMIN, when the duration is too short. This may cause large errors on the voltage at low speed.